1. Technical Field
The present invention relates to a technique for manufacturing polysilicon, and more particularly, to a technique for manufacturing high purity polysilicon having a purity of 99.9999% (6N) from metal-level raw silicon having a purity of 99 to 99.9% (2˜3N) using electron-beam melting.
2. Description of the Related Art
Purity of silicon is typically represented by 2N, 3N, 6N, 11N, and the like. Here, the numeral in front of ‘N’ means the number of nines (9) in percent by weight (wt %). For example, 2N means a purity of 99%, 6N means a purity of 99.9999%, and 11N means a purity of 99.999999999%.
Semiconductor level silicon requires an ultra-high purity approaching 11N. However, as known in the art, silicon used as a raw material for photovoltaic cells and having a relatively low purity of 5N to 7N provides similar optical conversion efficiency to silicon having a high purity of 11N.
Semiconductor level silicon is produced through chemical gasification processes. However, such a silicon production process generates a large amount of contaminants, and has low production efficiency in spite of high production cost.
Thus, the silicon production process is not suited to production of semiconductor level silicon used as a raw material for photovoltaic cells, and efforts for developing metallurgical refining processes capable of achieving mass production of high purity silicon at low cost have been actively attempted.
Examples of metallurgical refining processes for production of high purity silicon for photovoltaic cells include a vacuum refining process, a wet refining process, an oxidation process, a unidirectional solidification refining process, and the like. Some of these refining processes are commercially used in the art.
Particularly, silicon manufacturing techniques based on metal melting such as vacuum refining and unidirectional solidification refining have been actively studied due to merits thereof, such as easy characteristic control and less contamination due to impurities during operation.
Here, the vacuum refining process is a process in which a raw metal is melted and impurities having lower boiling point and vapor pressure than silicon are removed from the molten metal, and the unidirectional solidification refining process is a process in which impurities are moved (segregated) along an interface between solid and liquid during phase change of silicon from liquid to solid.
Various types of vacuum and segregation refining processes according to energy sources have been developed, and most refining processes employ magnetic induction heating.